With rapid development of the customer-side short-distance interconnection field in recent years, an increasingly high requirement is raised for a system capacity, and a pure non-return-to-zero code (NRZ) system cannot meet a requirement. A format of a four-level pulse amplitude modulation (PAM4) code gains increasingly more attention. The PAM4 code is a solution to 200 G/400 G communication within 500 meters (m) to 10 kilometers (km) or even to 40 km. A current research on PAM4 has evolved from a standard stage to a practical stage, and many manufacturers are developing high-performance PAM4 chips and devices.
The PAM4 code includes four levels, and a quantity of levels of the PAM4 code is doubled relative to that of an NRZ signal. Therefore, a requirement on device linearity is increased. In an actual application, the four levels need to be evenly distributed to achieve optimal judgment. Therefore, a requirement on device linearity is relatively high. In addition, a baud rate of a PAM4 signal is decreased by a half relative to that of a binary on-off keying (OOK) signal in a case of a same bit rate, thereby reducing a requirement on a bandwidth of a device. Relative to a coherent system, the PAM4 code does not need a complex coding/decoding algorithm or a decorrelation algorithm, bringing remarkable advantages in power consumption and costs.
In the prior art, there are two main PAM4 signal generation technologies.
I. A PAM4 signal generation method based on an electro-absorption modulated laser (EML): Currently, a PAM4 generation device based on a single-ended EML is relatively common; in this case, the EML may be integrated in a transmitter optical subassembly (TOSA), and a current mainstream technology is to generate a PAM4 signal based on an EML, for example, a 25 GHz EML is used to generate a PAM4 signal with 26.56 Gbauds. However, the PAM4 raises a relatively high linearity requirement for a system. Therefore, a device of higher linearity generally obtains a PAM4 signal of higher quality. However, it is not quite easy to design a high-linearity TOSA device.
From a perspective of a working principle of a single-ended EML, a cut-off area is formed in an area of a relatively low voltage, and an absorption coefficient of an electro-absorption (EA) modulator of the EML no longer changes with a reduction in a voltage. A saturation area is formed in an area of a relatively high voltage, and similarly, an absorption coefficient of the EA modulator no longer changes with an increase in a voltage. This characteristic of the EML is quite conducive to an NRZ signal modulation code, may inhibit a level 0 and a level 1 of an NRZ signal, and is conducive to improving NRZ signal quality. However, this characteristic is quite detrimental to the PAM4 signal because the PAM4 signal is generated in a linear area of a modulation curve. It is quite difficult to form a relatively good linear area due to impact of the characteristic of the single-ended EML itself. Therefore, it is quite difficult for the single-ended EML to generate a high-quality PAM4 signal.
In addition, for a radio frequency amplifier that drives the EML, if a PAM4 electrical signal is directly used to drive the EML to generate a PAM4 optical signal, a quite high linearity requirement is raised for an electrical device required in this solution, and as a modulation rate increases, an increasingly high requirement is raised for an optical device and an electrical device that are required for PAM4 signal modulation.
II. A PAM4 signal generation method based on a directly modulated laser (DML): Linearity of the DML is superior to that of an EML, but this does not mean that a PAM4 signal generated based on the DML is of quite good performance because the DML-based PAM4 signal has a performance loss in another aspect. For example: 1. A difference between characteristics of high and low temperatures: Because there is no temperature control unit in the DML, the characteristics of the high and low temperatures are deteriorated quite severely. In a case of a high temperature, not only a bandwidth of the device is quickly degraded, but also linearity is degraded, exerting relatively great impact on performance. Even if there is a DML device with a temperature control unit, a bandwidth of the DML device is still a bottleneck in an engineering application. 2. A chirp effect: Because the DML directly adjusts a current, a chirp effect may be accompanied in a signal modulation process. This is detrimental to a long-distance application scenario. In addition, the chirp effect of the laser and a chromatic dispersion effect of an optical fiber interact with each other, causing relatively high chromatic dispersion. 3. Currently, it is relatively difficult to design a high-linearity driver for the DML. Therefore, a high-quality PAM4 signal cannot be generated based on the DML.
It can be known from the foregoing that, in the prior art, the single-ended EML cannot generate a PAM4 signal of relatively high quality due to a linearity limitation, and the single-ended DML cannot generate a PAM4 signal of relatively high quality either due to impact of the characteristics of the high and low temperatures and a noise characteristic that are of the single-ended DML device.